1. Field of the Invention
The present invention relates to an afocal optical system in which focal points of optical elements each having a finite focal length coincide with each other at a predetermined point. The present invention also relates to a multibeam recording apparatus comprising the afocal optical system.
2. Description of the Background Art
FIG. 1 is a diagram of a conventional afocal optical system which is known in the art as a Keplerian type beam expander. The beam expander is comprised of two positive power lenses L31 and L32 which are spaced away from each other by a distance (f31+f32), where f31 is a focal length of the lens L31 and f32 is a focal length of the lens L32. The focal point of the lens L31 and the focal point of the lens L32 coincide with each other at a predetermined point A. Hence, a light beam LB1 parallel to the optical axis Z entering the beam expander would be converted into a light beam LB3 which leaves the beam expander parallel to the optical axis Z, the light beam LB3 satisfying: ##EQU1## where hi is a height of the incident light beam LB1 taken from the optical axis Z; hi' is a height of the leaving light beam LB3 taken from the optical axis Z; and m is a magnification of the beam expander.
FIG. 2 is a diagram showing other example of a conventional afocal optical system and illustrates the relationship between an object 202 and an image 204. The afocal optical system is comprised of a lens L41 having a focal length f41 and a lens L42 having a focal length f42, which are disposed a distance (f41+f42) away from each other. The image of the object 202 placed the distance f41 in front of (on the left side of) the lens L41 is obtained at a point the distance f42 behind (on the right side of) the lens L42.
The beam expander of FIG. 1 needs to comprise a larger lens L32 if the increase in diameter of light beam LB3 is desired. Likewise, the both side telecentric optical system of FIG. 2, which is telecentric on both the image and the object sides, needs to comprise a larger lens L42 if a larger image 204 is desired.
When reduction in size of these optical systems (shortened optical path) is desired, one of the approaches to attain is to shorten the focal length f32 of the lens L32, for example. However, such would result in reduction in the F-number of the optical system, which in turn would require an increased number of lenses to be used to achieve an optical system which is capable of carrying out the same optical performance as the optical system of FIG. 1. Thus, although the optical path is shortened, the number of lenses which form the optical system is increased, and hence, the manufacturing cost and the weight of the optical system are increased.
Conversely, if the optical system is formed by less lenses to place priority on the number of the lenses to form the optical system, the optical performances of the optical system would be deteriorated, creating various aberrations. Since the optical system can no longer satisfy the relation hi'=m.multidot.hi due to the aberrations, the optical system is not reliable enough to be applied to an optical apparatus such as a beam expander and a multibeam recording apparatus.
When the optical system can not satisfy the relation hi'=m.multidot.hi, chances are that even if the light beam LB1 impinging upon the afocal optical system is parallel to the optical axis Z, the light beam LB3 leaving the optical system is not parallel to the optical axis Z. In such likely case, if the optical system is used in an apparatus which requires a telecentric characteristic especially on the imaging side, an image would be distorted.
FIG. 3 is a diagram of a conventional multibeam recording apparatus. In FIG. 3, the multibeam recording apparatus comprises a plurality of light source parts which are arranged at equal intervals (only one light source part 12 is shown in FIG. 3), a reduction optical system 200 which is formed by lenses L20 and L21, a zoom lens 32 which is formed by lenses L22 to L24, and an afocal optical system 34 which is formed by lenses L25 and L26.
The light source part 12 includes a semiconductor laser 14. A laser beam from the semiconductor laser 14 is collimated by a collimating lens 16, and then pass through an aperture 18 to be allowed to the reduction optical system 200 parallel to the optical axis Z. The reduction optical system 200 has the same structure as that of the conventional afocal optical system of FIG. 2. That is, as shown in FIG. 3, the rear focal point of the lens L20 coincides with the front focal point of the lens L21, and therefore, the reduction optical system 200 is an afocal optical system. The laser beams from the reduction optical system 200 are magnified at a proper magnification by the zoom lens 32, focused by the afocal optical system 34 at the focal plane FP3 of the afocal optical system 34, and irradiated onto a recording surface RS which is disposed at the focal plane FP3 of the afocal optical system 34. Since principal rays of the laser beams are each perpendicular to the focal plane FP3, a magnification does not change even when a distance between the focal plane FP3 and the recording surface RS is changed. Thus, highly accurate image drawing is attainable.
Laser beams from the other light source parts which are not shown are irradiated onto the recording surface RS in a similar manner so that a plurality of beam spots are formed at the same time on the recording surface RS.
Constructed as above, the conventional multibeam recording apparatus needs a larger lens in order to increase the number of the beam spots which are formed on the recording surface RS at one time, i.e., the number of the channels. As can be understood from FIG. 3, to obtain more channels, more light source parts 12 disposed in a direction perpendicular to the optical axis Z are necessary, and therefore, the lens L20 must be enlarged accordingly at the expense of deteriorated aberration at the lens L20 and increased costs for manufacturing the lens L20.
On the other hand, to obtain a smaller multibeam recording apparatus by reducing the size of the optical system of FIG. 3, the focal length f0 of the lens L20 and hence the optical path must be shortened. However, when the focal length f0 is reduced, the F-number of the optical system will become smaller. In such a case, an increased number of lenses must be used to ensure the same optical performance which are obtainable from the optical system of FIG. 3. As a result, although the optical path is shortened, the number of the lenses which form the optical system, and hence, the manufacturing costs and the weight of the optical system are increased.
Conversely, if the optical system is formed by less lenses to place priority on the number of the lenses to form the optical system, the optical performances of the optical system would be deteriorated, creating various aberrations. Hence, although the light source parts 12 are arranged equidistant from each other, spacings between adjacent beam spots which are irradiated through the optical system onto the recording surface RS, that is, the beam pitches, will become uneven or the configurations of the beam spots will be deformed. Further, since the principal rays of the laser beams striking the focal plane FP3 are not perpendicular to the focal plane FP3, with a change in a distance between the focal plane FP3 and the recording surface RS, the magnification of the optical system (beam pitches) will be changed. A result of this is degraded quality of a recorded image.